Electric power from the wind will always need high capital investment because the density of the medium is so low. Better generating equipment and standardized design has made wind power using horizontal axis turbines with some subsidies competitive to coal and gas power plants. The cost of wind generated electricity is now between 0.03 to 0.08 dollars per kilowatt hour. This cost needs to be reduced to 0.03 dollars or less to create the critical mass that would overcome the “technological momentum” of the coal, gas and oil based power. One advantage is wind power does not contribute to the greenhouse effect caused by the burning of hydrocarbons that produce CO2.
Water currents are also being harnessed to produce electrical power. The energy in the currents is kinetic rather than potential. In a typical tidal stream installation, a turbine might be placed underwater to extract the energy. It's an underwater equivalent of a wind power generator.
The state of the art wind power turbines have in the past 10 years has taken giant leaps. This was accomplished by scaling-up the wind turbines to immense sizes. Rotor diameters on some of the new turbines have reached 400 ft and tower height of 600 ft. The reason that large diameter is more desirable is because the power goes up per the square of the rotor diameter. These titans of the wind industry generate 5 MW each. The rotational speed per minute is about the same as wind speed thus the speed of the rotor tip has reached a maximum above which acoustic noise is an undesirable byproduct. This immense size has reached a plateau so that they can not be scaled up by further increasing blade diameter.
Another limiting factor is that large amount of land is used because wind farms need to spread out so that one rotor does not diminish the power of the adjacent rotors. Wind turbines need to be separated by up to seven rotor diameters. Some new wind farms now require 21,000 acres near population centers that can use the power they produce. They are sometimes being planed near resort towns and are unsightly even as they are located out at sea.
Wind often doesn't blow when needed. The demand for electricity is continuous but variable while energy from the wind is intermittent and variable thus supply and demand seldom matches. The turbines must feed the electric grid but they normally supply only a fraction of the total demand, the rest must still come from other sources. At other times supply exceeds demand. Denmark which can generate 20% of its power from wind often sells the access power at very low prices because the increase capacity on a windy day is not needed. Coal, gas and oil fired plants thus make up the bulk of electrical generating capacity and adding more wind generating capacity will not make economic sense. The market is then said to be saturated. When this happens or when the wind doesn't blow the large capital investment in wind power is idle and thus unproductive. Another deficiency is that the access energy is wasted because it is too expensive to store it for later use.
The typical horizontal-axis turbines under optimal wind conditions can operates at 59% or less of maximum available power. This is known as Betz limit. Statistics for turbines in Denmark is that they produce at 50% and as a rule of thumb 1000 kWh/m2 per year. The state of the art turbines are marvels of technology; they use the most advanced materials but are limited by Betz limit and the area of the rotor. The power tower on the other hand is not limited in size. It can operate at 90% of maximum available power and up to four times velocity cubed. The power towers can overcome most of the deficiencies of the horizontal-axis turbines, while at the same time using the same components. This is the machine that will power the new century.